The objective of this research is to provide a better understanding and basis for treatment of certain disturbances of cardiac rhythm and conduction in man. The major emphasis of the research will be on patients with disturbances of sinus node function, patients with the Wolff- Parkinson-White Syndrome and patients with abnormalities of intraventricular conduction. Carefully acquired clinical data incuding EGG and VCG will be obtained in each patient. The patients will then undergo a period of physiologic study to define more accurately the basis of their ECG and/or rhythm abnormality. These studies will involve recordings from the specialized conduction system, the analysis of responses to premature stimulation and the use of dose-response relations to characterize certain changes after the administration of drugs selected because of their anticipated action on each of the above conditions. Finally, many of these patients will undergo direct electrophysiological studies in the operating room. These will include maps of the sequence of excitation of the heart and the evaluation of the response to premature stimuli. Emphasis will be placed on patients who are candidates for surgery, either because of a condition such as congenital, rheumatic or coronary heart disease which is associated with the rhythm or conduction disturbance of interest, or because they are candidates for implantation of a pacemaker. In some of these patients we hope to test the hypothesis that surgical interventions may play a role specifically in the treatment of the rhythm disturbance or its consequences. We believe that a unique feature of this proposal is the coordination in the same patients and by the same investigators of the pre-operative and operative electrophysiologic studies.